This invention relates to the determination of location coordinates of devices embodying GPS sensors.
The NAVSTAR Global Positioning System (GPS) developed by the United States Department of Defense uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, which allows devices embodying GPS sensors to determine their current location. The initial application was predominantly for military purposes, namely weapons targeting and troop deployment. The first widespread consumer based application was navigational assistance. These early applications shared similar operating conditions in that the GPS navigational devices (also called GPS receivers) were (1) used outdoors, and (2) co-located with the end-user. Because of the requirement for mobility, GPS receivers were typically battery-operated devices, with power consumption a critical design consideration.
Today, a new wave of applications is emerging, requiring a wider operating environment, including indoor operation. The major sectors include, government and safety—(emergency location and E-911 services), enterprise and industrial (asset tracking and monitoring), and consumer (location based services). Because current GPS processing techniques are unable to provide the receive sensitivity required for reliable indoor operation, these applications have developed slowly. The major factors impacting indoor and “urban canyon” operation of GPS receivers are (1) path losses due to obstructions between the GPS satellites and the GPS receiver, (2) multi-path fading of the incoming GPS signal, and (3) the requirement to obtain pseudo ranges for a minimum of four GPS satellites in order to determine the three dimensional coordinates of the GPS receiver.
The signals from all the GPS satellites are broadcast using the same carrier frequency, 1.57 GHz in the case of the NAVSTAR system. However, each satellite has a unique identifier, or pseudorandom noise (PRN) code having 1023 chips, thereby enabling a GPS receiver to distinguish the GPS signal from one GPS satellite from the GPS signal from another GPS satellite. In addition, each satellite transmits information allowing the GPS receiver to determine the exact location of the satellite at a given time. The GPS receiver determines the distance (pseudo range) from each GPS satellite by determining the time delay of the received signal. The pseudo range information includes a local time offset to each GPS satellite from the time-of-arrival of the PRN code, the Zcount and ephemeris parameters in the GPS signal that it receives from that GPS satellite. The determination of three-dimensional location coordinates can be accomplished with as few as three satellite pseudo ranges, provided they are measured using a time reference. Since this is impractical with current GPS navigational platforms, the computation of location coordinates is generally accomplished using four pseudo ranges. This is illustrated in FIG. 1, where pseudo range information 15, 16, 17, 19 is used to determine the location coordinates of GPS receiver 10. Once the pseudo ranges for at least four GPS satellites have been determined, it is a straightforward process to determine the location coordinates of the GPS receiver.
FIG. 2 describes the data structure of the signal that is broadcast by each GPS satellite, where the signal contains 50 Hz data overlay signal—20 millisecond data bits modulating a one millisecond PRN code interval of 1023 bits or chips. The PRN code is known as a spreading code because it spreads the frequency spectrum of the GPS signal. This spread spectrum signal is known as a direct sequence spread spectrum (DSSS) signal.
Indoors, or in urban canyons, the satellite signals reach the receiver by multiple paths. The result is a signal that is the composite of multiple instances of the transmitted signal, each reduced in power and differentially delayed. All things being equal, the sensitivity of a receiver is effectively reduced in such a multi-path environment. In strong signal communications applications, adaptive equalization techniques have been employed to combat the effects of multi-path—to prevent, for example the destructive combination of multiple instances of the transmitted signal, delayed relative to each other. To date, no such technology has been developed for applications in which the signal is buried in noise, such as satellite positioning. So, whereas a GPS receiver out in the clear is likely to see a single instance of a given satellite transmission, indoors the receiver is likely to see multiple variously-attenuated instances, delayed relative to each other, as shown in FIG. 3. The impact of this can be confounding to the prior art GPS receivers described in the paragraphs which follow.
FIG. 4 illustrates a block diagram of a prior art GPS receiver. The GPS signal from GPS satellite constellation 36 is received by the R/F front end 31 of GPS receiver 30. R/F front end 31 down converts the 1.57 GHz R/F signal, resulting in an intermediate frequency (I/F) signal. The streaming I/F signal is examined by correlator 32, which employs a search algorithm to confirm the presence or absence, within the composite GPS signal, of a component signal from one of the GPS satellites. In a typical search algorithm, the local frequency 33 is scanned across a range of frequencies; for each frequency, a series of correlations involving the incoming GPS signal and all possible code phases of a local replica 34 of the designated satellite's PRN code are used to “acquire” (i.e., determine the presence or absence of) the designated satellite. In order to ensure that the correct code phase is not missed due to local clock off-set, it is conventional to increment the local replica code phase in one-half chip or even smaller steps. The granularity of these steps is limited by the amount of over sampling that is performed on the incoming I/F signal. A high correlation peak value indicates that the designated satellite is present. If no correlations peaks are high enough, the local frequency 33 is set to a second trial frequency and the correlations are repeated. Once pseudo range information has been obtained for at least four GPS satellites along with the corresponding satellite timing information, the coordinate generator 35 determines the three-dimensional location coordinates of the GPS receiver 30. There are a number of drawbacks to this approach, including a long time to first fix (TTFF) and reduced receive sensitivity in certain situations (e.g., those involving severe path loss or multi-path).
The TTFF for GPS receiver 30 generally includes (1) the time to acquire a GPS satellite by tuning the local frequency 33 and the code phase of the local PRN code replica 34 in the GPS receiver to match the carrier frequency and the PRN code phase of the designated satellite, (2) the time to receive data bits in the GPS signal to determine a GPS clock time, (3) the time to receive ephemeris parameters in the GPS data bits, and (4) the time to process the code phase timing, GPS clock time and ephemeris for determining a position.
In order to overcome the TTFF disadvantages of GPS receiver 30, a prior art GPS receiver 50, with GPS assistance system 59 has been introduced (see FIG. 5). The role of GPS assistance system 59 is to track, the satellites “acquirable” in the vicinity of GPS assistance system 59. Accordingly, the GPS signal from GPS satellite constellation 56 is received by the R/F front end 51 of GPS receiver 50. R/F front end 51 down converts the 1.57 GHz R/F signal, resulting in an intermediate frequency (I/F) signal. The streaming I/F signal is examined by correlator 52, which is used to acquire satellites. To expedite the acquisition process, frequency information derived by GPS assistance system 59 in the course of tracking acquirable satellites, is transmitted to GPS receiver 50. This information is used to set the local frequency used by the search algorithm of correlator 52, enabling the search algorithm to operate more efficiently and more effectively. As a result the TTFF is significantly reduced, and receive sensitivity is improved slightly at the margin. Once pseudo range information has been determined for a minimum of four GPS satellites, the location coordinates are determined by coordinate generator 55. In addition to frequency information, ephemeris parameters may be transmitted to GPS receiver 50 from GPS assistance system 59. This information, however, does not affect the receive sensitivity of GPS receiver 50.
To enable operation in indoor or urban canyon environments, GPS receivers must cope successfully with (1) path loss between the GPS satellites and the GPS receiver, (2) multi-path fading of the incoming GPS signal, and (3) the requirement to obtain the pseudo range information for at least four GPS satellites in order to determine the location coordinates of the GPS receiver. The ability of GPS receivers 30 and 50 to reliably obtain a GPS fix in indoor and urban canyon environments has been shown to be severely limited. The explanations relate in large measure to the signal processing techniques employed in current receivers. Consider first the fact that correlator 32 uses an iterative-search algorithm to acquire a particular GPS satellite from a streaming composite I/F signal. Furthermore, complicating the satellite acquisition process is the fact that the I/F signal carries a 50 Hz data overlay signal with bit boundaries occurring after 20 consecutive PRN code intervals. Consequently, the amount of data that the correlator is capable of examining in order to acquire a particular satellite is typically much less than 20 msec. The 50 Hz data overlay signal thus precludes processing gains that could otherwise be achieved by examining more of this highly repetitive data.
To surmount the barrier imposed by the 50 Hz data overlay and enable the correlator to examine substantially larger amounts of data is impractical within the envelope of a battery-powered handheld device. A third prior art GPS receiver, exemplified by Geotate's implementation of GPS receiver 30, as it were, on the Personal Computer, is unencumbered by these limitations, as it performs the iterative correlations in software, off-line. While it represents an interesting repartitioning of the basic GPS receiver, the rearchitecting essential to improve the sensitivity of the basic GPS receiver has not been addressed.
Since none of the prior art techniques are capable of providing reliable indoor and urban canyon operation, there is a need in the art for a method of improving the receive sensitivity of GPS-enabled devices, consistent with the requirements of the emerging E-911, asset management, and location-based consumer applications.